Positioning control apparatus

ABSTRACT

A polyphase synchronous motor is rotated by a unit incremental angle for each pulse input to the motor control circuit. The position error between the reference position and the controlled position of an object connected to the motor is detected. The direction of the motor rotation is controlled by the polarity of the detected position error, and the frequency of the input pulse to the motor control circuit is maintained to be proportional to the magnitude of the detected position error. But this frequency is limited in a predetermined range, when the magnitude of the detected position error exceeds the corresponding limit.

BACKGROUND OF THE INVENTION

This invention relates to a positioning control apparatus for asynchronous motor.

A positioning control apparatus for a synchronous motor was disclosed indetail by a U.S. Pat. No. 4,151,449 entitled "Positioning ControlApparatus" (hereafter will be called the prior invention) owned by thesame applicant of the present invention.

The positioning control apparatus disclosed by the prior inventionbelongs to an open-loop control system in which the control of asynchronous motor is performed in a way similar to that of a steppingmotor.

As is well known, there are several demerits which are inherent to anopen-loop positioning control system. For example, a momentary powerfailure may cause the evaporation of the memory in the register whichstores the coded signal representing the instantaneous angular positionφ, and then, when the power is restored, the motor may follow to anangular position indicated by the new contents of the register which hasno relation to the position φ.

A closed-loop positioning control apparatus for a synchronous motor wasalso disclosed in detail by a U.S. Pat. No. 4,051,419, entitled "ControlSystem of an Alternating-Current Motor" owned by the same applicant ofthis invention. But the control apparatus disclosed by the U.S. Pat. No.4,051,419 is rather complicated and is not adapted to be used for asmall-power synchronous motor. And the open-loop type control apparatusdisclosed by the prior invention can be transformed to a closed-typepositioning control apparatus when an error-detecting device issupplemented to the apparatus and the input pulse frequency is kept tobe proportional to the amount of the error signal.

It is, therefore, a principal object of the present invention to providea control apparatus whereby a closed-loop positioning control isattained by a simple attachment to the open-loop type control apparatusdisclosed by the prior invention.

These and other objects of the present invention will become manifestupon a study of the following description and the accompanying drawings.

SUMMARY OF THE INVENTION

In one illustrative embodiment of the present invention, a N-phasesynchronous motor is controlled, just in the same way as in the priorinvention, by controllable power supplies which supply instantaneousphase currents respectively proportional to cos φ, cos (φ-2πφN), . . .cos {φ-(k-1)2π/N}, . . . cos {φ-(N-1)2π/N} to each of the N phasewindings where φ is the reference electrical position angle of themotor. In the embodiment of the present invention, there is alsoprovided an up-down counter, just in the same way as in the priorinvention, which stores a digital code representing the angular positionφ. In the prior invention, an input pulse to the up-down counter meant aprimary command for a unit increment (or decrement) of the angularposition, and the motor simply followed to the command. In the presentinvention, however, the primary command is given as a reference angularposition θ_(i). The controlled angular position θ_(o) is detected andthe error angle θ_(e) =θ_(i) -θ_(o) controls the frequency of the inputpulse to the up-down counter. Thus, in the system disclosed by thepresent invention, the motor position is controlled in a closed systemto reduce the amount of θ_(e) =θ_(i) -θ_(o) to a minimum.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings in which:

FIG. 1 is a block diagram of an embodiment of this invention; and

FIG. 2 is a circuit diagram of controllable power supplies as shown by ablock in FIG. 1.

DESCRIPTION OF THE INVENTION

Referring now to FIG. 1 in which a block diagram of an embodiment of thepresent invention is illustrated, a polyphase synchronous motor 10drives and controls the position of a controlled apparatus (not shown inthe drawing) through the gear 81, 82. A shaft encoder 83 connected tothe gear 82 detects the controlled position θ_(o) of the controlledapparatus. The digital code representing the reference position isreceived on the input terminal 84. A subtractor circuit 85 operates analgebraic subtraction θ_(e) =θ_(i) -θ_(o) to generate the error anglecode θ_(e) and transmits the digital code representing the amount ofθ_(e) (|θ_(e) |) to the terminal 850 and transmits one bit coderepresenting the polarity of θ_(e) to the terminal 851. The output linesfrom the shaft encoder 83, those from the terminal 84, and those fromthe terminal 850 are respectively shown by a single line in FIG. 1 inorder to simplify the drawing. But it must be understood that as manylines as the number of parallel bits are required in the bit paralleloutput system. This simplified representation will also be employed inall the attached drawings. The data processor 86 in the embodiment shownin FIG. 1 is a limiter circuit which receives |θ_(e) | and transmitsE=E_(m) where |θ_(e) |≧E_(m') or transmits E=|θ_(e) | where |θ_(e)|<E_(m). An accumulator comprises the adder circuit 87, the latch 88 andthe clock pulse circuit 89. When the modulo of the accumulator isdenoted by Q, an overflow pulse representing the value Q is generated atthe carry terminal of the most significant bit stage at each time whenthe sum at the adder circuit 87 becomes not less than Q. The number ofthe overflow pulses M which is generated at the terminal 871 in onesecond is represented by an equation M=EF/Q; where E represents theoutput value from the limiter circuit 86 and F (Hz) represents theoutput frequency of the clock pulse circuit 89. It is assumed that theup-down counter 30 is a binary 8 bit counter and that the mostsignificant bit represents the angle of 2π/2¹ (radian), the leastsignificant bit will represent 2π/2⁸ (radian), and an overflow pulsewhich is supplied to the counter 20 through the terminal 31 correspondsto the angular increment of Δφ=2π/2⁸.

Where θ_(e) ≧0, the counter 30 is controlled, by the control input fromthe terminal 32, to up-count the input pulse from the terminal 871.Where θ_(e) <0, the counter 30 is controlled by the control input fromthe terminal 32, to down-count the input pulse from the terminal 871.

As will be described in a later paragraph, the angular position of thesynchronous motor 10 is controlled to be equal to the angular positionrepresented by the contents θ of the counter 30 and controls the angularposition θ_(o) of the shaft encoder 83 through the gears 81, 82. Whereθ_(e) (=θ_(i) -θ_(o))≧0, the counter 30 increases the contents φ as anupcounter whereby increasing the controlled angular position θ_(o).Where θ_(e) <0, the counter 30 decreases φ as a downcounter therebydecreasing the controlled angular position θ_(o) ; and where θ_(e) =0,there will be no overflow pulse from the adder circuit 87 and at thisangular position the motor 10 stops. Therefore the angular position ofthe motor 10 is feedback controlled in such a way as to make θ_(o)=θ_(i).

In the embodiment shown by FIG. 1, it was assumed that the dataprocessor 86 was a limiter circuit limiting the upper and the lowerlimit on the error angle θ_(e) which is generated from subtractorcircuit 85, for use as a manipulated variable E.

But, in a general feedback control system, it is usual to compose themanipulated variable Z by adding at least one control action selectedfrom an integral control action and a derivative control action in orderto improve the response characteristics of the feedback system. Thus,the manipulated variable Z is, in general, represented by an equation##EQU1## where p represents the differential operator d/dt, k₁, k₂, k₃,are constants (involving the value zero) to be determined by design. Atleast one of these three constants must not be zero. Therefore the dataprocessor 86 comprises, in general, the circuit for operating thecalculation shown by equation (1) and the limiter circuit for imposingthe upper and the lower limit on the manipulated variable Z. And in theembodiment shown in FIG. 1, it was further assumed that the upper andthe lower limit of θ_(e) have the same absolute value E_(m), and thedata processor 86 required only |θ_(e) | as the input. But, in general,the manipulated variable Z is limited in a region as indicated by Zmin≦Z≦Z max, where Z max>0, Z min<0, and Z min≠Z max. Therefore, it isnecessary that both the absolute value |θ_(e) | and the sign coderepresenting the polarity of |θ_(e) | must be received by the dataprocessor 86.

In the embodiment as shown in FIG. 1, the position error θ_(e) wasgenerated in a digital form by the subtractor circuit 85 which receivesthe output signal θ_(o) of the shaft encoder 83 and the referenceposition signal θ_(i) from the terminal 84.

But it is apparent that any heretofore known position error detector maybe employed in this invention; for example, when a synchro transmitterand the synchro controlled transformer are used, the position errorθ_(e) is detected as an analog amount, and then this analog amount maybe converted to a digital code by an A-D converter.

The controllable power supplies 70 are the same to those of the priorinvention, and an embodiment will be here described.

FIG. 2 is a circuit diagram showing an example of the controllable powersupplies as shown in FIG. 1. The same numerals in FIG. 2 and FIG. 1 showthe same or the corresponding part. In FIG. 2, the number N of thephases of the polyphase synchronous motor 10 is shown as 4 and the rotor12 is shown as a permanent magnet having a single pair of magneticpoles. A synchronous motor which has two stator windings 14 and 16spaced at right angle to each other, is generally called a two phasemotor. But the synchronous motor 10 in FIG. 2 may be considered as afour phase (N=4) motor in which the third phase stator winding is unitedwith the first phase stator winding 14 and the fourth phase statorwinding is united with the second phase stator winding 16. Thus, thethird phase current

    I cos {φ-2(2π/4)}=-I cos φ

flows in the direction opposite to the first phase current I cos φ, andthe fourth phase current ##EQU2## flows in the direction opposite to thesecond phase current ##EQU3##

A d.c. power source 62 receives a commercial frequency power supply fromthe terminal 60 and generates a positive d.c. voltage +Vm and a negatived.c. voltage -Vm of a same magnitude. Transistor chopper circuits 63,64, 65 and 66 are connected between the power source 62 and the statorwindings 14 and 16. Capacitors 67 and 68 are connected in parallel tothe windings 14 and 16 respectively.

A clock pulse generator 1 generates a clock pulse of a sufficiently highfrequency and a pulse counter 2 counts this clock pulse.

Coincidence circuits 23 and 24 detect the coincidence of the lower 7bits between the output of the counter 2 with the output of the latches35 and 36 respectively.

The ROM (Read Only Memory) 33 and the ROM 34 are addressed by thecontents φ of the up-down counter 30 and the magnitude and the sign ofsin φ and cos φ are respectively read out. The output of the ROM 33 istransferred to a latch 35 and the output of the ROM 34 is transferred toa latch 36 at a predetermined count-phase of the counter 2, by a pulsewhich is transmitted through the line 22. The latches 35, 36 transmitthe polarity sign of sin φ, cos φ through the lines 351 and 361respectively, and transmit the codes for |sin φ|, |cos φ| to thecoincidence circuits 23 and 24. The other sides of the coincidencecircuits 23 and 24 receive the contents of the counter 2.

The flipflops 25 and 26 are set by the overflow pulse generator eachtime when the count phase of the counter 2 becomes zero. It will beassumed that the frequency of the output pulse at the clock pulsegenerator 1 is 128 kHz and that the counter 2 is a seven bit binarycounter. The frequency of the overflow pulse from the counter 2 will be1 kHz. And when the count phase of the counter 2 is coincident to thevalue of |sin φ|, a pulse is generated from the coincidence circuit 24to reset the flipflop 26. In the same way, when the count phase of thecounter 2 is coincident to the value of |cos φ|, a pulse is generatedfrom the coincident circuit to reset the flipflop 25.

Therefore the time width during which the flipflop 25 or 26 remains setis respectively proportional to |cos φ| and |sin φ|.

When cos φ>0, the gate 41 keeps the transistor 63 conducting for aduration proportional to cos φ to flow the current in a positivedirection through the winding 14. When cos φ<1, the gate 43 keeps thetransistor 64 conducting for a duration proportional to |cos φ| to flowthe current in a negative direction through the winding 14. When sinφ>0, the gate 45 keeps the transistor 65 conducting for a durationproportional to |sin φ| to flow the current in a positive directionthrough the windings 16. And when sin φ<0 the gate 47 keeps transistor66 conducting for a duration proportional to |sin φ| to flow the currentin a negative direction through the winding 16.

Thus, the average magnitudes of the currents in the windings 14 and 16become proportional to cos φ, sin φ respectively.

It is obvious that these stator currents produce a stator magnetic fieldin the direction of the reference angular position φ, and the rotor 12stops at an angular position which corresponds to the reference angularposition φ.

In the embodiment shown in FIG. 2, the polyphase synchronous motor 10has two stator windings 14 and 16 spaced at right angle to each other.When a synchronous motor 10 is a three phase motor which has 3 statorwindings spaced at 120° separated from each other, three ROMs must beprovided for reading out the absolute values and the polarity code forcos φ, cos (φ-2π/3) and cos (φ-4π/3) when addressed by the contents φ ofthe up-down counter 30. The latches and the coincidence circuits mustalso be provided one each for each ROM to control the current of eachstator winding equal to I cos φ, I cos (φ-2π/3) and I cos (φ-4π/3)respectively.

Similarly, when the synchronous motor 10 is a N-phase synchronous motorwhich has N stator windings spaced at 2π/N to each other, thecontrollable power supplies 70 may be designed to flow the current ofmagnitude representing I cos φ, I cos (φ-2π/N), I cos {φ-2(2π/N)}, . . .I cos {φ(N-1) (2π/N)} through each stator windings.

It will be apparent that the clock pulse circuit 89 shown in FIG. 1 mayprocess the output pulse from the generator 1 (FIG. 2) to generate therequired clock pulse or may generate the appropriate frequency pulseindependently.

In the prior invention, the synchronous motor 10 is controlled in anopen-loop system by transmitting the count pulse to the terminal 31 ofthe up-down counter 30, the direction of the rotation of the motor 10being controlled by the signal to the terminal 32.

In the present invention, the position error detectors 83, 84 and 85,data processor 86 and the accumulator comprising 87, 88 and 89 aresupplemented to compose a feedback control system. Thus in case of amomentary power failure and the resultant disappearance of the contentsof the up-down counter 30, the synchronous motor 10 is feedbackcontrolled to make the error angle θ_(e) =θ_(i) -θ_(o) to a minimum whenpower supply recovers. Therefore there are no danger of controlling thesynchronous motor 10 to erroneous angular position in a momentary supplyfailure. Moreover, in the present invention, the responsecharacteristics of the control system can be easily improved by addingat least one control action selected from an integral control action anda derivative control action.

What I claim is:
 1. A positioning control apparatus comprising:apolyphase synchronous motor for controlling the position of an object;an error signal generator means for generating an error signalrepresenting the magnitude of the position error between a predeterminedreference position and the controlled position of said object and forgenerating a polarity signal representing the direction of the positionerror; a data processor means, connected to said error signal generatormeans, for generating a modification signal which is a function of theerror signal; a pulse generator means connected to said data processormeans, for generating pulses having a pulse repetition frequencyproportional to the magnitude of the modification signal; and a motorcontrol means for receiving the polarity signal and said pulsesgenerated by said pulse generator, and driving said polyphasesynchronous motor in a direction determined by said polarity signal andby a unit incremental angle for each one pulse of said pulses generatedby said pulse generator, wherein said motor control means comprises:anup-down counter for counting the pulses from the pulse generator, saidpulses being counted in a first direction when the polarity signal ispositive and counted in a second direction when the polarity signal isnegative; a polyphase cosine data generator means, connected to theoutput of said up-down counter, for generating cosine data signalsrespectively representing cos φ, cos (φ-2π/N), . . . cos {φ-(k-1) 2π/N},. . . cos {φ-(N-1) 2π/N} where φ is the output of the counter, N is thetotal number of phases of said polyphase synchronous motor, and K is thesequential order of a phase concerned; and a controllable power supplyfor supplying the polyphase stator windings of said motor, saidcontrollable power supply receiving the cosine data signal for the phaseconcerned from said polyphase cosine data generator means andcontrolling the current in said phase winding to maintain said currentat a value proportional to said cosine data represented by said receivedcosine data signal.
 2. A positioning control apparatus comprising:apolyphase synchronous motor for controlling the position of an object;an error signal generator means for generating an error signalrepresenting the magnitude of the position error between a predeterminedreference position and the controlled position of said object and forgenerating a polarity signal representing the direction of the positionerror; a data processor means, connected to said error signal generatormeans, for generating a modification signal which is a function of theerror signal, wherein said data processor means generates saidmodification signal in a range limited by an upper limit and a lowerlimit, said signal in said range being proportional ##EQU4## where prepresents the differential operator d/dt, k₁, k₂ and k₃ are constants,at least one of said constants being non-zero, and θe represents theerror signal; a pulse generator means connected to said data processormeans, for generating pulses having a pulse repetition frequencyproportional to the magnitude of the modification signal; and a motorcontrol means for receiving the polarity signal and said pulsesgenerated by said pulse generator, and driving said polyphasesynchronous motor in a direction determined by said polarity signal andby a unit incremental angle for each one pulse of said pulses generatedby said pulse generator.